Radiation thermometry enables remote and quick measurement of the temperature of heated objects in industrial environments as well as in scientific research.
Conventional methods for measuring the temperature of a surface by radiation thermometry require knowledge of the spectral emissivity of the heated object, i.e. the effectiveness of the surface to radiate compared to a blackbody or a perfect radiator. In many industrial processes, especially those involving metals, e.g. steel, aluminum, etc. and electronic materials, the surface spectral emissivity of the object is not known and often shows significant variation during processes such as rolling, forging, casting, cutting, etc. due to phase changes, surface alloying, surface roughing, etc. In many of these processes, accurate temperature control is necessary in order to achieve high productivity, high product quality and/or desired properties of the product. Because of the variation of the spectral emissivity during these processes, however, conventional radiation thermometry cannot be relied upon for true temperature measurement. As a result, conventional methods for measuring temperature by radiation thermometry have been modified in an attempt to minimize spectral emissivity uncertainty or variations. These methods can be classified into two groups (1) "hybrid" methods which require the use of auxiliary devices such as mirrors, external radiation sources, etc. and (2) "compensation" methods, wherein the spectral emissivity is prescribed by some function based upon apriori or assumed information about the target material.
The "hybrid" methods are complicated by the need to install auxiliary equipment in the vicinity of the target material. In many industrial operations, the environment is harsh, placing severe requirements on the equipment. Besides being costly, this method generally requires sophisticated radiometric analysis and in-mill testing to evaluate. Furthermore, the method is usually restricted to a very specific or limited application.
Conventional "compensation" methods have been demonstrated for very specific situations but have not been adapted to industrial practice. One common compensation method, spectral or single-color pyrometry, is based on the assumption of constant emissivity of an object. It is known that accurate temperature measurement by single-color pyrometry cannot be expected when applied to a material whose surface is undergoing changes.
Another common compensation method, ratio pyrometry, assumes a gray body condition for which the ratio of two spectral emissivities is assumed constant and/or known. However, since the emissivity is regarded as a very complex function of wavelength, temperature, surface roughness, chemical composition of material, degree of oxidation or alloying, etc., most materials show a complicated variation in the ratio of two emissivities and thus ratio pyrometry results in unreliable temperature measurement when applied to a material undergoing surface changes.
In recent years, multispectral methods, which use more than two wavelengths, have been studied seriously. These methods apply various expressions of emissivity as a function of wavelength. For example, using an emissivity-wavelength linear relationship, it is possible to predict emissivities using exact or least-squares techniques and thus infer surface temperatures. Such a technique is cumbersome and the outcome is reliable only for stable materials such as noble metals and ceramics. A further disadvantage of these methods is the requirement for three or more wavelengths which seriously complicates instrumentation for an industrial environment.